Optical fiber cable

ABSTRACT

An optical fiber cable for blown installation has a tubular central strength member about which tubes loosely housing optical fibers are stranded. The jacket is preferably formed from a high density polyethylene material and is extruded over the core on which rip cords are positioned. A water blocking compound is provided in each tube in any spaces therein which would otherwise be void. Also a water blocking material is provided between the sheath and the layer defined by the helically wound wrapping tape or binding yam in any spaces between the tubes which would otherwise be void.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/GB01/03343, filed Jul. 25, 2001, the content of which isincorporated herein by reference, and claims the priority of EuropeanPatent Application No. 00306704.8, filed Aug. 7, 2000, the content ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical fibre cable and more particularlyalthough not exclusively to such cable for blown installation.

2. Description of the Related Art

One method of installing an optical fibre cable in a duct comprisesblowing the cable into the duct using gaseous flow which is fed into aninlet end of the duct together with the cable. The length of cable whichcan be blown into the duct can be increased by applying an additionalpushing force to the cable at the inlet end of the duct. Optical fibrecables for blown installation do not have to withstand high tensileloads (unlike cables which are to be pulled through ducting) but they dorequire a degree of stiffness for the application of the pushing force.

A design of optical fibre cable marketed by Pirelli Cables Ltd under thedesignation ‘Multi-Element Loose Tube’ (MLT) is shown in FIG. 1 and hasa core 10 comprising a plurality of tubes 12 stranded about a centralsolid strength member 14 and a plurality of optical fibres 16 in eachtube 12, and a jacket 18 surrounding said core 10. An aluminium/plasticslaminate tape 20 surrounds the tubes to retain the same about thestrength member 14 and also to act as a moisture barrier and, because itis electrically conductive, to enable location of the cable to bedetected. Also spaces within each tube which would otherwise be void arefilled with a water blocking material 22.

SUMMARY OF THE INVENTION

One object of the invention is to provide an optical fibre cable forblown installation with improved installation performance.

To this end the present invention provides an optical fibre cable havinga core comprising at least one tube stranded about a central strengthmember and at least one optical fibre loosely housed in said at leastone tube, and a jacket surrounding said core, wherein said centralstrength member is tubular, said central strength member having aperipheral wall enclosing a passage extending along the length of saidcentral strength member.

The at least one tube stranded about the central strength member neednot contact the central strength member. Thus a sheath may be disposedbetween said central strength member and said at least one tube.

The central strength member may be formed in a fibre reinforced plasticsmaterial, for example a glass fibre reinforced plastics material.

At least one elongate element enabling cable location detection may beaccommodated within and extend along the length of said passage of thecentral strength member.

Alternatively or additionally at least one tube member loosely housingat least one optical fibre may be accommodated within and extend alongthe length of said passage of the central strength member.

Alternatively or additionally at least one empty tube member for blownoptical fibre installation may be accommodated and extend along thelength of said passage of the central strength member.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be well understood some embodimentsthereof which are given by way of example only will now be describedwith reference to the accompanying drawings in which:

FIG. 1 is a radial cross-section of a known optical fibre suitable forblown installation; and

FIGS. 2 to 5 are respective radial cross-sections of four optical fibrecables for blown installation which embody the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The cables illustrated in FIGS. 2 to 5 each have a core 100 comprising aplurality of polymeric tubes 112 stranded about a central strengthmember 114 and a plurality of optical fibres 116 in each tube 112, and apolyethylene jacket 118 surrounding the core 100. In each cable, thecentral strength member 114 is tubular—the central strength memberhaving a peripheral wall 119 enclosing a passage 120 extending along thelength of the strength member 114.

A polymeric sheath 121 is disposed between the central strength member114 and the tubes 112.

The tubular central strength member 114, which typically has an outerdiameter of 3 to 8 mm and a wall thickness of 0.75 to 2.5 mm, is formedof a fibre reinforced plastics material such as a glass fibre reinforcedplastics material. However, resin matrices such as carbon and aramidcomposites may also be used for the central strength member. Thestrength member 114 may be formed with aramid yarn in a stranded layer.It is also envisaged that an engineering polymer per se may be used.Further the strength member may be a thin wall metal tube.

Helically wound wrapping tape 122 surrounds the tubes 112 to retain thetubes 112 which are preferably stranded with an alternating, orso-called S-Z lay in position against the sheath 121 duringmanufacturing and handling of the core. Instead of the wrapping tape, abinding yarn may be used. The wrapping tape may be made from paper orplastics. Suitable materials include polyethylene, polyester, waterswellable composite structures and metallic film structures. Typicalyarns include polyester, polypropylene and polyethylene.

The jacket 118 is preferably formed from a high density polyethylenematerial and is extruded over the core 100 on which rip cords 124 arepositioned. Water blocking compound 126 is provided in each tube 112 inany spaces therein which would otherwise be void. Also a water blockingmaterial 128 is provided between the sheath 121 and the layer defined bythe helically wound wrapping tape or binding yarn in any spaces betweenthe tubes 112 which would otherwise be void.

It will be noted that a difference between the known design of cableillustrated in FIG. 1 and the cables embodying the invention illustratedin FIGS. 2 to 5 is that the central strength member 14 in FIG. 1 issolid, ie formed as a rod whereas the central strength member 114 ineach of the embodiments shown in FIGS. 2 to 5 is tubular. The provisionof a tubular central strength member instead of a solid central strengthmember reduces the weight per unit length of the cable whilst increasingits stiffness against radial deformation on application of a pushingforce by a caterpillar device or the like at the inlet end of the ductinto which the cable is being installed about its central axis. Thedecrease in weight and the increase in stiffness each increase thedistance to which the cable may be blown.

FIG. 2 shows a cable in which the passage 120 of the tubular centralstrength member 114 is left void.

FIG. 3 shows a cable in which a tube 130 loosely housing a plurality ofoptical fibres 132 is accommodated within and extends along the lengthof the passage 120. Preferably any spaces within the tube 130 whichwould otherwise be void are filled with a water blocking compound

As will be appreciated, the cable of FIG. 3 has a higher number ofoptical fibres than that of FIG. 2 with no increase in outside diameterof the cable through its utilisation of the passage 120.

FIG. 4 shows a cable in which empty tubes 140 are accommodated withinand extend along the length of the passageway. Each empty tube 140 isable to have an optical fibre blown therein to increase the number ofoptical fibres in the cable without increasing its outside diameter.

FIG. 5 shows a cable in which a detectable elongate element 150 isaccommodated within and extends along the length of the passage 120.This element enables the location of the cable when it is buried orotherwise concealed to be determined. The element 150 preferablycomprises an electrically conductive element such as a copper wire ortwisted pair.

In the cables shown in FIGS. 2 to 5 the tubular central strength member114 is preferably manufactured by pulltrusion with a cross head feed forthe introduction into the passage 120 of the tube 130 of loosely housedoptical fibres 132 in the case of FIG. 3, the empty tubes 140 in thecase of FIG. 4 or the detectable elongate element 150 in the case ofFIG. 5. Alternatively, the tube 130, the tube 140 or the element 150 maybe blown into the passage 120 after manufacture of the tubular strengthmember 114.

In the cables shown in FIGS. 2 to 5, the aluminium/plastics laminatetape 20 of the cable shown in FIG. 1 has been omitted. As stated above,this tape is provided in the cable of FIG. 1 to act as a moisturebarrier and to enable the location of the cable to be detected.Replacement of this tape 20 with paper or plastics tape 122 or bindingyarn reduces the weight of the cable enabling the distance the cable canbe blown into a duct of the same internal diameter to be increased.

Also the presence of the layer of aluminium/plastics laminate providesthe cable with a shape memory which hinders unwinding of the cable froma drum during installation, the unwound cable being blown into the ducttending to take a spiral rather than straight form. Accordinglyreplacement of the aluminium/plastics laminate tape 20 with paper orplastics tape 122 or binding yarn improves the unwinding characteristicsof the cable and thus increases the distance the cable can be blown.

However, it is to be understood that in the embodiments of FIGS. 2, 3and 4 the aluminium/plastics laminate tape 20 may be used instead oftape 122 to enable the location of the cable to be determined

Alternatively the embodiments of FIGS. 2, 3 and 4 may be modified toinclude an elongate element 150 within and extending along the passage120 for cable location detection.

Whilst a plurality of tubes 112 (shown as twelve) are provided in thecables of FIGS. 2 to 5, it is to be understood that these cables may beprovided with more or less tubes 112 and in an extreme case with onlyone such tube 112. Similarly, whilst each tube has a plurality ofoptical fibres (shown as twelve) loosely housed therein it is to beunderstood that more or less fibres and in an extreme case only one suchoptical fibre maybe provided within the or each tube 112.

1. An optical fibre cable having a core comprising at least one tubestranded about a central strength member and at least one optical fibreloosely housed in said at least one tube, and a jacket surrounding saidcore, wherein said central strength member is tubular, said centralstrength member having a peripheral wall enclosing a passage extendingalong the length of said central strength member, the passage beingconfigured to receive a tube member by blown installation, the tubemember loosely housing a plurality of optical fibres.
 2. An opticalfibre cable having a core comprising at least one tube stranded about acentral strength member and at least one optical fibre loosely housed insaid at least one tube, and a jacket surrounding said core, herein saidcentral strength member is tubular, said central strength member havinga peripheral wall enclosing a passage extending along the length of saidcentral strength member and a tube member extending along said passage,the tube member being configured to receive an optical fibre by blowninstallation and to loosely house the optical fibre.
 3. An optical fibrecable having a core comprising at least one tube stranded about acentral strength member and at least one optical fibre loosely housed insaid at least one tube, and a jacket surrounding said core, wherein saidcentral strength member is tubular, said central strength member havinga peripheral wall enclosing a passage extending along the length of saidcentral strength member and a tube member extending along said passageand loosely housing a plurality of optical fibres.
 4. An optical fibrecable having a core comprising at least one tube stranded about acentral strength member and at least one optical fibre loosely housed insaid at least one tube, and a jacket surrounding said core, wherein saidcentral strength member is tubular, said central strength member havinga peripheral wall enclosing a passage extending along the length of saidcentral strength member, the passage including at least one empty tubemember extending along the length of said passage of the centralstrength member.
 5. The optical fibre cable as claimed in any one ofclaims 1 to 4, further comprising a sheath disposed between said centralstrength member and said at least one tube.
 6. The optical fibre cableas claimed in any one of claim 1 to 4, wherein said central strengthmember is formed in a fibre reinforced plastics material.
 7. The opticalfibre cable as claimed in any one of claims 1 to 4, further comprisingat least one elongate element enabling cable location detectionaccommodated within and extending along the length of said passage ofthe central strength member.